U.S. patent number 6,577,088 [Application Number 09/966,673] was granted by the patent office on 2003-06-10 for closed loop spindle motor acceleration control in a disc drive.
This patent grant is currently assigned to Seagate Technology LLC. Invention is credited to Jeffrey A. Heydt, David R. Street.
United States Patent |
6,577,088 |
Heydt , et al. |
June 10, 2003 |
Closed loop spindle motor acceleration control in a disc drive
Abstract
Method and apparatus for accelerating a disc drive spindle motor
from rest to a final operational velocity. During a low gear mode,
the spindle motor is accelerated from rest to a first velocity
through application of fixed duration drive pulses to the spindle
motor. A high gear mode is next employed to accelerate the spindle
motor from a first velocity to an intermediate velocity. Variable
duration drive pulses are applied to the spindle motor each having
a duration selected as a percentage of the duration of the most
recently detected commutation period of the spindle motor. Once the
spindle motor reaches the medium speed, commutation circuitry and
back electromotive force (bemf) detection circuitry use detected
bemf from the spindle motor to electronically commutate the motor
to accelerate to the final operational speed.
Inventors: |
Heydt; Jeffrey A. (Oklahoma
City, OK), Street; David R. (Oklahoma City, OK) |
Assignee: |
Seagate Technology LLC (Scotts
Valley, CA)
|
Family
ID: |
26952645 |
Appl.
No.: |
09/966,673 |
Filed: |
September 28, 2001 |
Current U.S.
Class: |
318/400.34;
318/400.09; 318/778; 318/798; G9B/19.027 |
Current CPC
Class: |
G11B
19/20 (20130101) |
Current International
Class: |
G11B
19/20 (20060101); A02K 023/00 () |
Field of
Search: |
;318/254,138,439,430,778,798,809,806,473 ;360/73.03 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Masih; Karen
Attorney, Agent or Firm: Fellers, Snider, et al.
Parent Case Text
RELATED APPLICATIONS
This application claims priority to Provisional Application No.
60/267,794 filed Feb. 9, 2001.
Claims
What is claimed is:
1. A method for accelerating a brushless direct current (dc)
spindle motor of a disc drive from rest to an operational velocity
comprising steps of: (a) initially accelerating the spindle motor
from rest to a first velocity by applying fixed duration drive
pulses to the spindle motor and detecting successive spindle motor
commutation state transitions as the spindle motor rotates through
a range of commutation states; (b) subsequently accelerating the
spindle motor from the first velocity to an intermediate velocity
less than the operational velocity by applying variable duration
drive pulses to the spindle motor and detecting successive spindle
motor commutation state transitions without the use of back
electromotive force (bemf) detected from the spindle motor, wherein
the variable duration of each successive drive pulse is established
in relation to a most recent commutation period comprising the
elapsed time between the two most recently detected state
transitions, and wherein a single one of the variable duration
drive pulses is applied during each commutation period; and (c)
accelerating the spindle motor from the intermediate velocity to
the operational velocity using bemf detected from the spindle
motor.
2. The method of claim 1, wherein the disc drive comprises back
electromagnetic force (bemf) detection circuitry which detects bemf
from rotation of the spindle motor above the intermediate velocity
and commutation circuitry which electrically commutates the spindle
motor in relation to the detected bemf over the range of
commutation states, and wherein step (c) comprises a step of using
the bemf detection circuitry and the commutation circuitry to
accelerate the spindle motor from the intermediate velocity to the
operational velocity in relation to the bemf detected from the
spindle motor.
3. The method of claim 1, wherein step (a) comprises steps of: (a1)
identifying the initial commutation state of the spindle motor
while the spindle motor is at rest; (a2) applying a fixed duration
drive pulse to the spindle motor to rotate the spindle motor; (a3)
repetitively measuring electrical rotational position of the
spindle motor until a transition to the next commutation state is
detected; and (a4) repeating steps (a2) and (a3) until the first
velocity is reached.
4. The method of claim 3, wherein the electrical rotational
position of the spindle motor is determined during step (a3) by
steps of: (a3i) sequentially applying sense pulses to the spindle
motor; and (a3ii) measuring a corresponding rise time for a
resulting voltage induced by application of each said sense pulse,
said rise time determined in relation to impedance of the spindle
motor determined in turn by the electrical rotational position of
the spindle motor.
5. The method of claim 1, wherein step (b) comprises steps of: (b1)
measuring the most recent commutation period; (b2) calculating a
drive pulse duration in relation to the most recent commutation
period and a scale factor so that the drive pulse duration is less
than the most recent commutation period; (b3) applying a drive
pulse with the calculated drive pulse duration of step (b2) to the
spindle motor; (b4) repetitively measuring electrical rotational
position of the spindle motor until a transition to the next
commutation state is detected; and (b5) repeating steps (b1)
through (b4) until the intermediate velocity is reached.
6. The method of claim 5, wherein the electrical rotational
position of the spindle motor is determined during step (b4) by
steps of: (b4i) sequentially applying sense pulses to the spindle
motor; and (b4ii) measuring a corresponding rise time for a
resulting voltage induced by application of each said sense pulse,
said rise time determined in relation to impedance of the spindle
motor determined in turn by the electrical rotational position of
the spindle motor.
7. The method of claim 5, wherein the scale factor used to
calculate the drive pulse duration during step (b2) is a
constant.
8. The method of claim 5, wherein the scale factor used to
calculate the drive pulse duration during step (b2) varies in
relation to the rotational velocity of the spindle motor as the
spindle motor accelerates from the first velocity to the
intermediate velocity.
9. A disc drive, comprising: a brushless direct current (dc)
spindle motor configured to rotate at least one recording disc; a
read/write head configured to write data to the disc and read data
from the disc as the spindle motor is rotated at a final
operational velocity; back electromagnetic force (bemf) detection
circuitry coupled to the spindle motor and which detects bemf from
rotation of the spindle motor above a medium velocity, the medium
velocity less than the final operational velocity; commutation
circuitry coupled to the bemf detection circuitry and spindle motor
which electrically commutates the spindle motor in relation to the
detected bemf over a range of commutation states of the spindle
motor; and a control circuit which accelerates the spindle motor by
applying variable duration drive pulses to the spindle motor and
measuring electrical rotational position to detect successive
spindle motor commutation state transitions without relying upon
bemf from the spindle motor, wherein the variable duration of each
successive drive pulse is established in relation to a most recent
commutation period comprising the elapsed time between the two most
recently detected state transitions, and wherein each of the
variable duration drive pulses is applied during a different
commutation period.
10. The disc drive of claim 9, wherein the control circuit
subsequently directs the bemf detection circuitry and the
commutation circuitry to accelerate the spindle motor from the
medium velocity to the final operational velocity in relation to
the detected bemf from the spindle motor.
11. The disc drive of claim 9, further comprising spindle driver
circuitry coupled to the spindle motor and the commutation
circuitry and which applies drive pulses to each of a plurality of
windings of the spindle motor in response to commutation pulses
from the commutation circuitry to rotate the spindle motor, wherein
the control circuit identifies the electrical rotational position
of the spindle motor by directing the spindle driver to
sequentially apply a sense pulse to each of a plurality of windings
of the spindle motor and measuring a corresponding rise time for a
resulting voltage induced by inductance of said windings in
relation to the electrical rotational position of the spindle
motor.
12. The disc drive of claim 11, wherein the spindle driver
circuitry comprises: a sense resistor connected in series between
the plurality of windings of the spindle motor and ground; and a
comparator having a positive input and a negative input, the
positive input connected in parallel with the sense resistor to
receive a motor sense voltage and the negative input connected to
receive a reference threshold voltage, wherein the comparator
outputs a pulse when the motor sense voltage reaches the reference
threshold voltage, and wherein the control circuit determines the
rise time in relation to the pulse output by the comparator.
13. The disc drive of claim 9, wherein the control circuit
comprises control logic coupled to the commutation circuitry and
the bemf detection circuitry, and a top level programmable
processor coupled to the control logic and which further directs
the transfer of data between the disc and the host device.
14. A disc drive, comprising: a spindle motor configured to rotate
at least one recording disc; a read/write head configured to write
data to the disc and read data from the disc as the spindle motor
is rotated at a final operational velocity; back electromagnetic
force (bemf) detection circuitry coupled to the spindle motor and
which detects bemf from rotation of the spindle motor above a
medium velocity, the medium velocity less than the final
operational velocity; commutation circuitry coupled to the bemf
detection circuitry and spindle motor which electrically commutates
the spindle motor in relation to the detected bemf over a range of
electrical rotational positions of the spindle motor; and first
means for accelerating the spindle motor by applying drive pulses
to the spindle motor and measuring electrical rotational position
to detect successive spindle motor commutation state transitions,
each said drive pulse having a variable duration established in
relation to a most recent commutation period comprising the elapsed
time between the two most recently detected state transition, each
said drive pulse applied during a different commutation period.
15. The disc drive of claim 14, wherein the first means comprises
control logic coupled to the commutation circuitry and the bemf
detection circuitry, and a top level programmable processor coupled
to the control logic and which further directs the transfer of data
between the disc and the host device.
Description
FIELD OF THE INVENTION
The claimed invention relates generally to the field of disc drive
data storage devices and more particularly, but not by way of
limitation, to a method and apparatus for accelerating a disc drive
spindle motor from rest to a final operational velocity.
BACKGROUND
A disc drive is a data storage device used to store digital data. A
typical disc drive includes a number of rotatable magnetic
recording discs which are axially aligned and mounted to a spindle
motor for rotation at a high constant velocity. A corresponding
array of read/write heads access tracks defined on the respective
disc surfaces to write data to and to read data from the discs.
Disc drive spindle motors are typically provided with a
three-phase, direct current (dc) brushless motor configuration. The
phase windings are arranged about a stationary stator on a number
of radially distributed poles. A rotatable spindle motor hub is
provided with a number of circumferentially extending permanent
magnets in close proximity to the poles. Application of current to
the windings induces electromagnetic fields which interact with the
magnetic fields of the magnets to apply torque to the spindle motor
hub and induce rotation of the discs.
Due to the prevalence of numerous consumer devices that employ
electrical motors, it might seem at first glance that accelerating
a disc drive spindle motor from rest to a final operational
velocity would be relatively straightforward; simply turn on the
motor and let it accelerate to the final desired speed. As those
skilled in the art will appreciate, however, just the opposite has
proven to be the case. Accelerating a spindle motor from rest can
be fraught with difficulty and involves a number of important
considerations that must be adequately taken into account.
First, it is important to accurately determine the rotational state
of a disc drive spindle motor prior to application of drive signals
to the motor. Application of drive signals to a spindle motor while
the motor is in an unknown state could lead to the inadvertent
rotation of the motor in the wrong direction. Rotating the spindle
motor in the wrong direction, even for a very short time, can lead
to premature failure of a disc drive; heads and disc surfaces can
be damaged, and lubricating fluid used in hydrodynamic spindle
motor bearings can be pumped out of the bearings.
Early disc drive spindle motor designs used Hall effect or similar
external sensors to provide an independent indication of motor
positional orientation. However, present designs avoid such
external sensors and instead use electronic commutation and back
electromagnetic force (bemf) detection circuitry to provide
closed-loop spindle motor control, such as discussed in U.S. Pat.
No. 5,631,999 issued to Dinsmore. Such approach generally entails
applying a predetermined sequence of commutation steps to the phase
windings of the spindle motor over each electrical revolution
(period) of the motor. A commutation step involves supplying the
motor with current to one phase, sinking current from another
phase, and holding a third phase at a high impedance in an
unenergized state.
Detection circuitry measures the bemf generated on the unenergized
phase, compares this voltage to the voltage at a center tap of the
windings, and outputs a signal at a zero crossing of the voltages;
that is, when the bemf voltage changes polarity with respect to the
voltage at the center tap. The point at which the zero crossing
occurs is then used as a reference for the timing of the next
commutation pulse, as well as a reference to indicate the position
and relative speed of the motor.
Above an intermediate operational speed, the control circuitry will
generally be able to reliably detect the bemf from rotation of the
spindle motor, and will further be able to use the detected bemf to
accelerate the motor to a final operational velocity. Below this
intermediate speed, however, closed-loop motor speed control using
detected bemf generally cannot be used since the spindle motor will
not generate sufficient bemf at such lower speeds.
Thus, a related difficulty encountered in accelerating a disc drive
spindle motor from rest is getting the motor to properly and safely
rotate up to the intermediate velocity so that the closed-loop
motor control circuitry can take over and accelerate the motor up
to the operational velocity.
Several approaches have been proposed in the prior art to
accelerate a disc drive spindle motor from rest to an intermediate
velocity, such as exemplified by U.S. Pat. No. 5,117,165 issued to
Cassat et al. This reference generally discloses determining the
electrical rotational position of a spindle motor to determine the
commutation state of the motor; that is, to determine the
appropriate commutation pulses that should be applied to accelerate
the motor based on the then-existing motor position. Drive pulses
of fixed duration are applied to the motor to induce torque and
initiate rotation of the motor, and the electrical rotational
position of the motor is measured between application of each
successively applied, fixed duration pulse.
Once the motor rotates sufficiently to induce a change in
commutation state, the next set of drive pulses are applied, and
position measurements are taken between the application of each set
of the drive pulses as before. As the motor achieves a higher
rotational velocity due to the successive "nudging" provided by the
drive pulses, the time between successive commutation states
becomes shorter, decreasing the number of drive pulses applied
during each commutation state.
Eventually, an upper limit on the achievable rotational velocity
will be encountered using this approach. This upper limit is
generally reached as the combined time for the drive pulses and
position measurement approaches one half the commutation time. As
the motor velocity approaches this upper limit, an uneven, cogging
action will typically be induced in the motor because the drive
pulses are not synchronized with the motor rotation; that is, the
drive pulses are not applied when the windings and magnets are
optimally aligned for each new commutation state. Such operation
does not generally harm the motor, but does result in less than
efficient operation and limits the torque that can be applied to
the motor. This cogging action ultimately acts as a velocity
governor and undesirably induces variation in the rotational
velocity of the motor.
The final velocity achieved by this approach must be high enough to
enable a hand off to the motor control circuitry; that is, the
final velocity must be high enough to enable the spindle motor to
generate bemf that can be detected and used by the bemf detection
circuitry. However, the particular velocity at which bemf is
reliably generated is a function of the motor construction, and
recent generation high performance spindle motor designs with
higher operational velocities and fewer numbers of poles have been
found to require a higher intermediate velocity before sufficient
bemf is generated to allow frequency lock by the motor control
circuitry.
Moreover, the motor speed variation increases as the motor velocity
reaches the upper limit, and such variation makes it more difficult
for the motor control circuitry to obtain frequency lock on the
spindle motor. Thus, as disc drive manufacturers implement spindle
motor designs with ever higher levels of performance, it is
becoming increasingly difficult for prior art motor start up
routines to accelerate the spindle motors to a sufficient velocity
to enable the motor control circuitry to take over and implement
closed-loop acceleration up to the operational velocity.
Accordingly, there is a need for improvements in the art whereby a
high performance spindle motor can be reliably accelerated from
rest to an operational velocity. It is to such improvements that
the present invention is directed.
SUMMARY OF THE INVENTION
In accordance with preferred embodiments, a disc drive includes a
spindle motor, back electromagnetic force (bemf) detection
circuitry which detects bemf from rotation of the spindle motor
above an intermediate velocity, commutation circuitry which
electrically commutates the spindle motor in relation to the
detected bemf over a range of commutation states, and control
circuitry which directs the acceleration of the spindle motor from
rest to a final operational velocity.
During a low gear mode, the spindle motor is initially accelerated
from rest to a first velocity by applying short, fixed duration
drive pulses to the spindle motor. Each drive pulse is preferably
followed by two quick position measurements. The drive pulses and
measurements continue until a commutation transition is detected,
after which a new set of drive pulses appropriate for the new
commutation state (and position measurements) are applied.
Once the first velocity is reached, a high gear mode is employed
wherein the spindle motor is accelerated from the first velocity to
an intermediate velocity greater than the first velocity. Variable
duration drive pulses are applied to the spindle motor and
successive spindle motor commutation state transitions are
detected. The variable duration of each successive drive pulse is
established in relation to a most recent commutation period
comprising the elapsed time between the two most recently detected
state transitions. Only one variable duration drive pulse is
preferably applied during each commutation state, after which
position measurements are repeatedly made while the spindle motor
coasts to the next state transition.
Thereafter, the spindle motor is accelerated from the intermediate
velocity to the final operational velocity using the commutation
circuitry and bemf detection circuitry. Zero crossings are detected
in relation to bemf from the spindle motor and the zero crossings
are used to time the application of subsequent commutation pulses
to the motor.
Preferably, operation during low gear mode includes steps of
identifying the initial commutation state of the spindle motor
while the spindle motor is at rest, and repetitively applying a
fixed duration drive pulse and measuring electrical rotational
position of the spindle motor until a transition to the next
commutation state is detected. The drive pulses and measurements
are repeated until the first velocity is reached.
Operation during high gear mode preferably includes steps of
measuring the duration of the most recent commutation period,
calculating a drive pulse duration in relation to the duration of
the most recent commutation period and a scale factor so that the
drive pulse duration is less than the duration of the most recent
commutation period, applying a drive pulse with the calculated
drive pulse duration to the spindle motor, and repetitively
measuring electrical rotational position of the spindle motor until
a transition to the next commutation state is detected. The
foregoing steps are repeated until the intermediate velocity is
reached. The scale factor can be a constant, or can vary in
relation to variations in the rotational velocity of the spindle
motor. Once the intermediate velocity is reached, the spindle motor
is accelerated to the operational velocity using back electromotive
force (bemf) detection.
By accelerating the spindle motor in this manner, smooth and
continuous transitions in spindle motor velocity are obtained, and
cogging and reverse rotation of the spindle motor are avoided.
These and various other features and advantages which characterize
preferred embodiments of the present invention will be apparent
from a reading of the following detailed description and a review
of the associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top plan view of a disc drive constructed in accordance
with preferred embodiments of the present invention.
FIG. 2 provides a functional block diagram of the disc drive of
FIG. 1.
FIG. 3 provides a functional block diagram of relevant portions of
the motor control circuitry of FIG. 2.
FIG. 4 provides a schematic representation of rotor position sense
(RPS) circuitry of the motor control circuitry of FIG. 3.
FIG. 5 is a flow chart for a SPINDLE MOTOR START UP routine
illustrative of steps carried out in accordance with preferred
embodiments of the present invention to accelerate the spindle
motor from rest to a first velocity, from the first velocity to an
intermediate velocity, and then from the intermediate velocity to
an operational velocity.
FIG. 6 is a graphical representation of a sensed voltage and a
corresponding timing measurement obtained using the RPS circuitry
of FIG. 4 during the steps of the flow chart of FIG. 5.
FIG. 7 is a graphical representation of rise times versus
electrical position for each commutation state of the spindle
motor.
FIG. 8 is a graphical representation of differences in rise times
(delta rise times) versus electrical position for each commutation
state.
FIG. 9 provides a timing diagram to illustrate operation of the
disc drive during low gear mode.
FIG. 10 provides a timing diagram to illustrate operation of the
disc drive during high gear mode.
FIG. 11 provides a generalized graphical representation of a motor
velocity profile obtained during acceleration of a spindle motor in
accordance with the prior art.
FIG. 12 provides a generalized graphical representation of a motor
velocity profile obtained in accordance with preferred embodiments
of the present invention.
DETAILED DESCRIPTION
FIG. 1 provides a top plan view of a disc drive 100 constructed in
accordance with preferred embodiments of the present invention. A
base deck 102 and a top cover 104 (shown in partial cutaway)
cooperate to form a sealed housing for the disc drive 100. A
spindle motor 106 rotates a number of magnetic recording discs 108
in a rotational direction 109. An actuator assembly 110 supports an
array of read/write heads 112 adjacent the respective disc
surfaces. The actuator assembly 110 is rotated through the
application of current to an actuator coil 114 of a voice coil
motor (VCM) 116.
FIG. 2 provides a functional block diagram of the disc drive 100.
FIG. 2 includes control circuitry provided on a disc drive printed
circuit board (PCB) affixed to the underside of the HDA 101, and
thus not visible in FIG. 1.
Data and host commands are provided from a host device to the disc
drive 100 using interface (I/F) circuitry 118 in conjunction with a
top level control processor 120. Data are transferred between the
discs 108 and the host device using the I/F circuitry 118, a
read/write (R/W) channel 122, and a preamplifier/driver (preamp)
circuit 124.
Head positional control is provided by a closed-loop servo circuit
126 comprising demodulation (demod) circuitry 128, a servo
processor 130 (preferably comprising a digital signal processor, or
DSP) and motor control circuitry 132. The motor control circuitry
132 applies drive currents to the actuator coil 114 to rotate the
actuator 110. The motor control circuitry 132 further applies drive
signals to the spindle motor 106 to rotate the discs 108.
FIG. 3 provides a functional block diagram of relevant portions of
the motor control circuitry 132 of FIG. 2. Control logic 134
receives commands from and outputs state data to the DSP 130.
Spindle driver circuitry 136 applies drive currents to the phases
of the spindle motor 106 over a number of sequential commutation
steps to rotate the motor. During each commutation step, current is
applied to one phase, sunk from another phase, and a third phase is
held at a high impedance in an unenergized state.
Back electromagnetic force (bemf) detection circuitry 138 measures
the bemf generated on the unenergized phase, compares this voltage
to the voltage at a center tap, and outputs a zero crossing (ZX)
signal when the bemf voltage changes polarity with respect to the
voltage at the center tap. A commutation circuit 140 uses the ZX
signals to time the application of the next commutation step.
The spindle driver circuitry 136 includes rotor position sense
(RPS) circuitry 142 to detect electrical position of the spindle
motor 106 in a manner to be discussed shortly. At this point it
will be noted, with reference to FIG. 4, that the RPS circuitry 142
includes a sense resistor RS 144, a digital to analog converter
(DAC) 146 and a comparator 148. FIG. 4 also shows the spindle
driver circuitry 136 to include six field effect transistors (FETs)
150, 152, 154, 156, 158 and 160, with inputs denoted as AH (A
high), AL (A low), BH, BL, CH and CL, respectively. Controlled,
timed application of drive currents to the various FETs result in
flow of current through A, B and C phase windings 162, 164 and 166
from a voltage source 168 to V.sub.M node 170, through the RS sense
resistor 144 to reference node (ground) 172. Spindle motor
commutation steps (states) are defined in Table 1:
TABLE 1 Commutation Phase Held at State Source Phase Sink Phase
High Impedance 1 A B C 2 A C B 3 B C A 4 B A C 5 C A B 6 C B A
During commutation step 1, phase A (winding 162) is supplied with
current, phase B (winding 164) outputs (sinks) current, and phase C
(winding 166) is held at high impedance. This is accomplished by
selectively turning on AH FET 150 and BL FET 156, and turning off
AL FET 152, BH FET 154, CH FET 158 and CL FET 160. In this way,
current flows from source 168, through AH FET 150, through A phase
winding 162, through the center tap (CT node 174), through B phase
winding 164, through BL FET 156 to V.sub.M node 170, and through RS
sense resistor 144 to ground 172. The resulting current flow
through the A and B phase windings 162, 164 induce electromagnetic
fields which interact with a corresponding array of permanent
magnets (not shown) mounted to the rotor (spindle motor hub), thus
inducing a torque upon the spindle motor hub in the desired
rotational direction. The appropriate FETs are sequentially
selected to achieve the remaining commutation states shown in Table
1.
It will be noted that each cycle through the six commutation states
of Table 1 comprises one electrical revolution of the motor. The
number of electrical revolutions in a physical, mechanical
revolution of the spindle motor is determined by the number of
poles. With 3 phases, a 12 pole motor will have four electrical
revolutions for each mechanical revolution of the spindle
motor.
The frequency at which the spindle motor 106 is commutated,
referred to as the communication frequency FCOM, is determined as
follows:
A three-phase, 12 pole spindle motor operated at 15,000 revolutions
per minute would produce a commutation frequency of:
or 9 kHz. The commutation circuit 140 will thus commutate the
spindle driver 136 at nominally this frequency to maintain the
spindle motor 106 at the desired operational velocity of 15,000
rpm. The foregoing relations can be used to determine the actual
motor speed (and therefore speed error) in relation to the
frequency at which the zero crossing ZX pulses are provided from
the bemf detection circuitry 138.
Having concluded a review of relevant circuitry of the disc drive
100, reference is now made to FIG. 5 which provides a flow chart
for a SPINDLE MOTOR START UP routine 200 illustrative of steps
carried out by the disc drive 100 in accordance with preferred
embodiments of the present invention to accelerate the spindle
motor 106 from rest to a final operational velocity.
More particularly, as explained below the routine accelerates the
spindle motor from rest to a first velocity in a "low gear" mode,
accelerates the motor from the first velocity to an intermediate
velocity in a "high gear" mode, and then finally accelerates the
motor from the intermediate velocity to the operational velocity
using bemf control. For clarity, the first velocity is defined as a
relatively low velocity of the motor. The intermediate velocity is
defined as a medium velocity between the low velocity and the
operational velocity, with the intermediate velocity high enough to
enable the spindle motor to generate bemf at a sufficient level for
the bemf detection circuitry to reliably detect the bemf and output
zero crossing (ZX) signals. The operational velocity is the final
velocity at which the spindle motor is normally operated during
data transfer operations.
These respective velocities can take any number of relative values
depending on the particular application, and are generally related
to the specific construction of the spindle motor. For purposes of
the present discussion, exemplary values are about 250 revolutions
per minute (rpm) for the first velocity, about 1,000 rpm for the
intermediate velocity and about 15,000 rpm for the operational
velocity.
The routine initially proceeds to determine the electrical
rotational position of the spindle motor 106. At step 202, the
motor driver circuitry 132 applies sense pulses to all six
commutation states and uses the RPS circuitry 142 to measure the
associated rise time of the voltage at V.sub.M node 170. The sense
pulses are of small enough amplitude and duration so as to not
impart a torque to the spindle motor sufficient to induce movement
of the motor, but the pulses are provided with sufficient amplitude
and duration to enable detection of the electrical position of the
spindle motor 106. FIG. 6 provides a graphical illustration of the
operation of this step.
FIG. 6 provides a V.sub.M voltage signal curve 210 and a
corresponding Is pulse curve 212, plotted against a common elapsed
time x-axis 214 and a common amplitude y-axis 216. Using the
commutation state 1 discussed above by way of example, a small
duration pulse is applied by the controlled activation of AH and BL
FETs 150, 156 (FIG. 4). A timer 217 (preferably disposed in the
control logic block 134, FIG. 3) is initiated at this time (time
t.sub.0). A suitable digital value for a threshold T is input to
the DAC 146 (FIG. 4, also preferably by the control logic 134). The
resulting voltage at V.sub.M node 170 will rise in relation to the
impedance of the spindle motor 106, and the impedance of the
spindle motor 106 will vary depending upon the electrical position
of the motor.
The comparator 148 is configured to receive the V.sub.M voltage at
its +input and the (analog equivalent) threshold voltage T at its
-input. When the V.sub.M voltage eventually reaches the threshold
level T (as indicated by broken threshold line 218 in FIG. 6), the
comparator will output an I.sub.S pulse 220, as shown in FIG. 6.
The receipt of the I.sub.S pulse will cause the control logic 134
to stop the timer (time t.sub.1), report the elapsed time between
t.sub.0 and t.sub.1, and instruct the spindle driver circuitry 136
to cease further application of the drive pulse (i.e., AH and BL
FETs 150, 156 are turned off). For clarity, the remaining shape of
the voltage V.sub.M after time t.sub.1 is shown in FIG. 6 in broken
line fashion at 222, to illustrate what voltage would have
continued to have been observed at the V.sub.M node 170 had the
pulse not been truncated by the control logic 134.
The foregoing operation is thus performed during step 202 for each
of the six commutation states in turn, and an elapsed time (time
t.sub.0 to t.sub.1) is obtained for each of the six commutation
states. The routine of FIG. 5 passes to step 224 where the elapsed
times are used to detect the electrical position of the spindle
motor 106. FIGS. 7 and 8 have been provided to illustrate how this
is preferably accomplished.
FIG. 7 provides a graphical representation of rise time versus
electrical (rotational) position for each of the six commutation
states of Table 1. Particularly, FIG. 7 provides substantially
sinusoidal curves 231, 232, 233, 234, 235 and 236 plotted against
position x-axis 238 and rise time y-axis 240. The curves are
normalized over a range of +1 to -1 and correspond to the six
commutation states of Table 1 as follows: curve 231 represents the
normalized rise time for commutation state 1, curve 232 corresponds
to commutation state 2, and so on. The curves are complimentary in
nature: state 1 (curve 231) is the complement of state 4 (curve
234); state 2 is the complement of state 5 (curve 235); and state 3
is the complement of state 6 (curve 236). This is to be expected
since, as shown by Table 1, the commutation of step 1 (application
of current to phase A and the sinking of current from phase B) is
the direct opposite of commutation step 4 (application of current
to phase B and the sinking of current from phase A).
The differences in rise times between each pair of complementary
states are plotted in FIG. 8, which provides delta rise time curves
241, 242 and 243 shown against electrical rotational position
x-axis 244 and normalized amplitude y-axis 246. It can readily be
seen from a comparison of FIGS. 7 and 8 that delta curve 241
represents the difference (delta) between states 1 and 4 (curves
231 and 234); delta curve 242 represents the difference between
states 2 and 5 (curves 232 and 235); and delta curve 243 represents
the difference between states 3 and 6 (curves 233 and 236).
It can further be seen from review of FIG. 8 that the electrical
position of the spindle motor 106 can be expressed as a three-digit
binary number corresponding to each of six zones of the commutation
cycle. For example, in the first zone, when the spindle motor 106
is in an electrical position of between zero and 60 degrees, the
values of the delta curves 241, 242 and 243 have corresponding
values of {01,0,0}. That is, the curve 241 has a positive value
greater than zero (which is assigned a value of one) and the curves
242, 243 have negative values less than zero (and are assigned
values of zero). As the spindle motor 106 rotates to a position
between 60 and 120 electrical degrees, the delta curve 242
(representing the difference between states 2 and 5) will
transition from a negative to a positive value, resulting in a
change of position to {01,1,0 }. As the spindle motor 106 continues
to rotate, one of the values in the three-digit binary position set
will transition from a zero to a one or vice versa as each
successive zone is encountered.
These observations are advantageous for at least two reasons:
first, the electrical position (zone) of the spindle motor 106 can
readily be determined in relation to the elapsed rise times for all
six commutation states. Calculations can be made in accordance with
the graphs of FIGS. 7 and 8, or predefined lookup tables can be
used to identify the electrical position from the measured values.
Second, once the electrical position (zone) of the spindle motor
106 is determined, subsequent measurements of electrical position
can be limited to just those two commutation states that will next
undergo a transition in delta value. For example, if the spindle
motor 106 is determined to be in the first zone, subsequent
measurements only need be made of states 2 and 5 to detect passage
of the spindle motor 106 into the second zone.
Returning to the routine of FIG. 5, the routine proceeds to slowly
accelerate the spindle motor 106 in the aforementioned low gear
mode. At step 248, a fixed duration drive pulse of is applied to
the spindle motor 106. This drive pulse is of sufficient duration
and amplitude to initiate rotation of the spindle motor hub in the
desired direction.
Immediately after application of the drive pulse at step 248, sense
pulses are applied at step 250 to the two appropriate states that
will change when the motor rotates into the next zone, and rise
times are measured for these two states in the manner described
above. The electrical position of the spindle motor 106 is detected
at step 252, and an inquiry is made at decision step 254 to
determine whether the motor has transitioned to the next
commutation state. If not, steps 250 and 252 are successively
repeated until this transition is detected, after which a new drive
pulse (of the same fixed duration as the first pulse) is applied to
the spindle motor 106 in relation to the new commutation state.
These operations are shown in the timing diagram of FIG. 9; the "D"
blocks correspond to the operation of step 248, and the measurement
"M" blocks correspond to the operation of steps 250, 252 and 254.
It will be noted that fewer "D" and "M" operations will be required
for each commutation state as the motor continues to accelerate,
and the rotational velocity of the motor can be determined in
relation to the elapsed time intervals between successively
detected commutation state transitions (commutation periods).
Once the spindle motor 106 reaches the first velocity, such as
around 250 rpm, the routine transitions to the high gear mode, as
indicated by decision step 256 in FIG. 5. The high gear mode is
used to accelerate the spindle motor 106 up to the intermediate
velocity (such as around 1000 rpm). The routine passes to step 258
where the duration of the most recently detected commutation
period, CP, is determined. At step 260, a drive pulse is applied to
the spindle motor 106 having a duration determined in relation to
the most recently detected commutation period, such as provided by
the following relation:
where n denotes each period, DP(n) is the duration of the drive
pulse for the nth period, CP(n-1) is the duration of the previous
commutation period, and K is a derating factor less than one. K can
be maintained constant, or can vary over time. For example, in one
embodiment the value of K is initially provided a value of 0.75 for
a motor velocity around 250 rpm, and this value is successively
reduced to a value of around 0.33 as the motor velocity approaches
the intermediate velocity (1,000 rpm).
At the conclusion of the drive pulse of step 260, sense pulses are
applied to the two appropriate states and resulting rise times are
measured at step 262. The rise times are used to detect electrical
rotational position at step 264. Decision step 266 inquires as to
whether a transition in commutation state has occurred. Once the
next transition has been detected, the most recent commutation
period is measured at step 268. This enables the motor control
circuitry 132 to determine the present velocity of the spindle
motor. If the velocity is less than the intermediate velocity, as
shown by decision step 270, the routine passes back to step 260 for
the application of the next drive pulse. These operations are
illustrated by the timing diagram of FIG. 10.
Once the spindle motor 106 reaches the intermediate velocity, the
motor control circuitry 132 passes from the high gear mode to
steady-state (normal) closed-loop control mode. The resulting hand
off in control is shown by passage of the routine from decision
step 270 to step 272, wherein the motor control circuitry 132
acquires frequency lock from the bemf detection circuitry 138 and
proceeds to accelerate the motor to the operational velocity (such
as about 15,000 rpm). The routine then terminates at step 274.
FIGS. 11 and 12 graphically illustrate the manner in which the
routine of FIG. 5 advantageously operates to accelerate the spindle
motor 106 from rest to the operational velocity. FIG. 11 shows a
prior art motor velocity curve 280 representative of data obtained
from a drive accelerated generally in accordance with the
methodology disclosed by the aforementioned Cassat U.S. Pat. No.
5,117,165 reference. The curve 280 is plotted against an elapsed
time x-axis 282 and a spindle motor velocity y-axis 284.
As evidenced by FIG. 11, the spindle motor incurred significant
variations as the spindle motor was accelerated from rest to the
intermediate velocity. Surprisingly, it was observed that under
certain circumstances the motor was actually caused to rotate for a
short time in the wrong direction during spin up.
By contrast, FIG. 12 shows a motor velocity curve 290
representative of data obtained from a drive accelerated in
accordance with the routine of FIG. 5. The velocity curve 290 shows
smooth and continuous transition from low gear to high gear mode
and from high gear mode to steady-state closed-loop control. A
faster overall acceleration to the operational velocity was also
achieved as compared to the prior art.
It will now be appreciated that the routine of FIG. 5 provides
several advantages over the prior art. One advantage is that only
one drive pulse is preferably applied to the spindle motor during
each commutation period of the high gear mode. These drive pulses
are preferably applied when the relative orientation of the rotor
and stator are such that substantially maximum torque efficiency is
achieved with each pulse.
Another advantage is the ability to use substantially longer
duration drive pulses as compared to the prior art during high gear
mode. This stems from a realization that, as the motor accelerates,
it can be reasonably predicted when the next commutation state
transition will occur. Thus, there is no need as in the prior art
to apply multiple short drive pulses and to take intervening
measurements while the motor is still rotating through the same
state and the next state transition is still relatively far away.
Rather, being able to generally predict when the next state
transition will occur allows substantially longer pulses to be
safely applied without fear of inadvertently continuing to apply a
drive pulse after the spindle motor transitions to a new state (and
thereby inducing undesirable cogging actions). The use of longer
pulses has been found to significantly reduce the time required to
accelerate the motor up to the intermediate velocity.
Another advantage is the amount of operational margin provided by
the routine of FIG. 5. Although the operation during high gear mode
does not rely on bemf detection from the spindle motor, the routine
nevertheless is able to detect, with good resolution, the
transition of the motor to each new state, and modify the duration
of subsequent drive pulses accordingly. While the routine of FIG. 5
will not generally be able to achieve a final velocity approaching
the final operational velocity achievable through bemf detection,
it has nevertheless been demonstrated that the routine of FIG. 5
can sustain a velocity well in excess of that necessary to enable
detection of the bemf from the spindle motor. Hence, it is
contemplated that the routine of FIG. 5 will find enhanced utility
with successive generations of higher speed spindle motors which
require higher motor speeds before frequency lock can take
place.
Finally, another advantage of the routine of FIG. 5 is the ability,
in certain circumstances such as head flight testing, to closely
maintain the rotation of the spindle motor at a desired velocity
less than the intermediate velocity. More particularly, FIG. 5
promotes a methodology whereby a spindle motor velocity of, for
example 500 rpm, can be readily maintained even though such level
is wholly insufficient to allow bemf control of the spindle motor.
This can be useful during engineering and manufacturing testing
operations.
Accordingly, it will now be understood that the present invention,
as embodied herein and as claimed below, is directed to a method
and apparatus for accelerating a disc drive spindle motor from rest
to an operational velocity. In accordance with preferred
embodiments, a disc drive (such as 100) includes a spindle motor
(such as 106), back electromagnetic force (bemf) detection
circuitry (such as 138) which detects bemf from rotation of the
spindle motor above a nominal rotational velocity, commutation
circuitry (such as 140) which electrically commutates the spindle
motor in relation to the detected bemf over a range of electrical
rotational positions, and control circuitry (such as 120, 134)
which controls the acceleration of the spindle motor.
During a low gear mode, the spindle motor is initially accelerated
from rest to a first velocity by applying fixed duration drive
pulses to the spindle motor (such as by step 248) and detecting
successive spindle motor commutation state transitions as the
spindle motor rotates through a range of commutation states (such
as by step 254).
Once the first velocity is reached, a high gear mode is employed
wherein the spindle motor is accelerated from the first velocity to
an intermediate velocity by applying variable duration drive pulses
to the spindle motor (such as by step 260) and detecting successive
spindle motor commutation state transitions (such as by step 266).
The variable duration of each successive drive pulse is established
in relation to a most recent commutation period comprising the
elapsed time between the two most recently detected state
transitions.
Thereafter, the spindle motor is accelerated from the intermediate
velocity to the operational velocity using the commutation
circuitry and bemf detection circuitry (such as by step 272).
Preferably, operation during low gear mode includes steps of
identifying the initial commutation state of the spindle motor
while the spindle motor is at rest (such as by steps 202, 224),
applying a fixed duration drive pulse to the spindle motor to
rotate the spindle motor (such as by step 248), and repetitively
measuring electrical rotational position of the spindle motor (such
as by steps 250, 252) until a transition to the next commutation
state is detected. The foregoing steps are repeated until the first
velocity is reached.
Operation during high gear mode preferably includes steps of
measuring the most recent commutation period (such as by step 258),
calculating a drive pulse duration in relation to the most recent
commutation period and a scale factor so that the drive pulse
duration is less than the most recent commutation period, applying
a drive pulse with the calculated drive pulse duration to the
spindle motor (such as by step 260), and repetitively measuring
electrical rotational position of the spindle motor (such as by
steps 262, 264) until a transition to the next commutation state is
detected. The foregoing steps are repeated until the intermediate
velocity is reached. The scale factor can be a constant, or can
vary in relation to variations in the rotational velocity of the
spindle motor.
For purposes of the appended claims, the function of the recited
"first means" element will be understood as being carried out by
the disclosed structure including the control logic (134, FIG. 3)
and the servo processor (130, FIG. 2) programmed in accordance with
the routine of FIG. 5. Prior art systems and methods that fail to
accelerate a spindle motor by applying drive pulses to the spindle
motor and measuring electrical rotational position to detect
successive spindle motor commutation state transitions, each said
drive pulse having a variable duration established in relation to a
most recent commutation period comprising the elapsed time between
the two most recently detected state transition, each said drive
pulse applied during a different commutation period, including the
aforementioned Duffy U.S. Pat. No. 5,631,999 and Cassat U.S. Pat.
No. 5,117,165 references are not encompassed by the element and are
further excluded from the definition of an equivalent.
It is to be understood that even though numerous characteristics
and advantages of various embodiments of the present invention have
been set forth in the foregoing description, together with details
of the structure and function thereof, this detailed description is
illustrative only, and changes may be made in detail, especially in
matters of structure and arrangement of parts within the principles
of the invention to the full extent indicated by the broad general
meaning of the terms in which the appended claims are expressed.
For example, the particular elements may vary depending on the
particular application for the motor start routine while
maintaining the same functionality without departing from the
spirit and scope of the invention.
In addition, although the embodiments described herein are
generally directed to a motor start routine for a disc drive, it
will be appreciated by those skilled in the art that the routine
can be used for other devices to accelerate a rotatable member from
rest without departing from the spirit and scope of the claimed
invention.
* * * * *